Engineering takeaway
- Use oil properties at the actual operating temperature, not ambient conditions.
- Density and specific heat determine circulation flow for a fixed duty and temperature rise.
- Viscosity changes Reynolds number, friction factor and system resistance.
- Pressure loss and flow determine the pump operating point and shaft power.
- Check both normal hot operation and cold startup.
1. Thermal oil is part of the design basis
Two heat-transfer oils can both be approved for service at 250–280°C and still behave differently inside the same heater. Their density, specific heat, viscosity and thermal conductivity may differ enough to change required circulation, tube velocity, Reynolds number, pressure drop, pump operating point and motor load.
Replacing an oil is therefore not only a purchasing or maintenance decision. The new fluid must be checked against the heat balance, heater-coil hydraulics, pump curve, controls and cold-start procedure. A heater designed around one property envelope may lose its intended operating margin after the fluid changes.
2. Four key thermophysical properties
For thermal and hydraulic calculations, four temperature-dependent properties form the core engineering dataset:
Density, ρ
Density converts mass flow to volumetric flow and appears directly in dynamic pressure. It also affects system inventory and expansion volume.
Specific heat, Cp
Specific heat defines the energy transported per kilogram per degree. Lower Cp requires more mass flow for the same duty and ΔT.
Viscosity, μ or ν
Dynamic viscosity μ and kinematic viscosity ν control Reynolds number and friction loss. Viscosity can rise dramatically during startup.
Thermal conductivity, k
Thermal conductivity contributes to heat transfer and tube-film temperature and must be assessed with velocity and viscosity.
Use temperature-dependent data
A property quoted at 20°C is not a design value for oil circulating at 260°C. Obtain manufacturer curves or validated correlations across the operating and startup range.
Other properties that matter
Oil selection also requires vapor pressure, maximum recommended bulk and film temperatures, flash point, fire point and oxidation stability. These properties influence expansion-tank pressure, vapor formation, safety margins, degradation rate and expected oil life even when they do not appear directly in the basic heat-balance equation.
3. How oil properties determine circulation flow
For sensible heating without phase change, the heater heat balance is:
Mass flow and volumetric flow are related by:
Combining both relationships gives:
For preliminary calculations, properties may be evaluated at the mean bulk oil temperature. For a wide temperature range, temperature-dependent properties should be integrated or evaluated using an appropriate engineering method.
For unchanged duty and allowable oil temperature rise, changing ρCp changes required volumetric flow. This changes pipe and coil velocity, which then influences both heat-transfer performance and hydraulic loss. The original pump duty cannot automatically be retained.
4. How oil properties affect heater pressure drop
The hydraulic effect begins with Reynolds number:
Pressure loss through straight pipe, heater coils, fittings and local restrictions may be represented by:
The chain is: oil properties change required flow, velocity and Reynolds number; Reynolds number changes friction factor; and density changes dynamic pressure directly. The combined result must be recalculated for the complete circuit.
Viscosity deserves special attention. During cold startup, the much higher viscosity can significantly increase friction loss and alter the system resistance characteristic. Pump flow, head and efficiency may also require viscosity correction, and the heater may not achieve the minimum coil velocity required to limit film temperature and oil degradation.
5. How this changes the pump operating point
A centrifugal pump operates where its pump curve intersects the system curve. When oil properties change required flow or resistance, that intersection moves. The pump may deliver insufficient circulation, operate away from its best-efficiency point or demand more power.
For an incompressible liquid of sufficiently low viscosity, a centrifugal pump's head curve in metres is approximately independent of density at a given speed, while the same curve expressed in pressure changes with density. For more viscous oils, corrections to flow, head and efficiency may be required. Calculations must therefore use consistent units and the pump manufacturer’s correction method where applicable.
If the circulation system uses flow control with a VFD to maintain a fixed flow setpoint, the controller may increase pump speed as system resistance increases. This can restore flow but raises required head and shaft power. Pump speed, motor load, NPSH margin and minimum heater velocity must remain acceptable.
6. Why pump motor sizing can become a problem
A pump selected with little motor margin for oil A may approach or exceed full load with oil B. The risk increases if the new oil needs more volumetric flow, produces greater circuit resistance or lowers pump efficiency. Cold-start viscosity can impose a higher temporary load than normal hot operation, so startup current and sequencing require a separate check.
7. Simplified illustrative example
Consider an existing heater where Total Seriola K3120 is replaced by YT-m30 while duty and target supply/return temperatures remain unchanged. Engineers should compare verified properties at each relevant temperature, recalculate flow from ρCp, update Reynolds number and friction loss, then overlay the revised system curve on the actual pump curve.
This is an illustrative engineering case, not a brand ranking. A dedicated case study will later document property sources, temperature basis, assumptions and the complete comparison.
8. One design chain for the complete system
9. Information engineers should request
- Manufacturer and exact oil product.
- Normal, minimum and maximum operating temperatures.
- Density, Cp, viscosity and thermal conductivity versus temperature.
- Vapor pressure versus temperature.
- Maximum recommended bulk and film temperatures.
- Flash point, fire point and current safety data.
- Oxidation stability, degradation and contamination limits.
- Cold-start properties and minimum startup temperature.
Conclusion
Thermal-oil selection connects the heat balance to every major hydraulic decision. Changing ρ, Cp, μ or k can change circulation flow, heat transfer, pressure loss, pump operating point and motor demand even when heater duty is unchanged. Treat the oil as a defined engineering input and revalidate the system whenever the fluid changes.